EventSubscription.cs

public class EventSubscription<TPayload> : IEventSubscription<TPayload>
{
    private readonly WeakReference<NotificationDelegate<TPayload>> _notificationRef;
    private readonly WeakReference<FilterDelegate<TPayload>> _filterRef;

    /// <summary>
    /// Initialises a new instance of <see cref="EventSubscriberBase{TEvent, TPayload}"/>
    /// </summary>
    /// <param name="token">The subscription token</param>
    /// <param name="notification">The notification delegate</param>
    /// <param name="filter">The filter delegate</param>
    public EventSubscription(
        SubscriptionToken token,
        NotificationDelegate<TPayload> notification,
        FilterDelegate<TPayload> filter = null)
    {
        Token = Ensure.IsNotNull(token, nameof(token));
        _notificationRef = new WeakReference<NotificationDelegate<TPayload>>(
            Ensure.IsNotNull(notification, nameof(notification)));

        filter = filter ?? ((c) => Task.FromResult(true));
        _filterRef = new WeakReference<FilterDelegate<TPayload>>(filter);
    }

    /// <inheritdoc />
    public FilterDelegate<TPayload> OnFilterAsync
    {
        get
        {
            if (_filterRef.TryGetTarget(out FilterDelegate<TPayload> filter))
            {
                return filter;
            }

            return null;
        }
    }

    /// <inheritdoc />
    public NotificationDelegate<TPayload> OnNotificationAsync
    {
        get
        {
            if (_notificationRef.TryGetTarget(out NotificationDelegate<TPayload> notification))
            {
                return notification;
            }

            return null;
        }
    }

    /// <inheritdoc />
    public SubscriptionToken Token { get; }
}

FilterDelegate.cs

public delegate Task<bool> FilterDelegate<TPayload>(EventContext<TPayload> context);

NotificationDelegate.cs

    public delegate Task NotificationDelegate<TPayload>(EventContext<TPayload> context);

SubscriptionToken.cs

public class SubscriptionToken : DisposableAction<SubscriptionToken>
{
    /// <summary>
    /// Initialises a new instance of <see cref="SubscriptionToken"/>
    /// </summary>
    /// <param name="unsubscribe">The unsubscribe action</param>
    public SubscriptionToken(Action<SubscriptionToken> unsubscribe)
        : base(unsubscribe) { }

    /// <summary>
    /// Get sthe token used to identify this subscription
    /// </summary>
    public Guid Token { get; } = Guid.NewGuid();
}

UnitTest.cs

[Fact]
public void OnNotification_ReturnsNullForDeadSubscriber()
{
    // Arrange
    var token = new SubscriptionToken(st => { });
    WeakReference weak = null;
    EventSubscription<object> subscription = null;

    Action act = () =>
    {
        NotificationDelegate<object> notification = c => Task.CompletedTask;
        weak = new WeakReference(notification);
        subscription = new EventSubscription<object>(token,
            (NotificationDelegate<object>)weak.Target);
    };

    // Act
    act();
    GC.Collect();
    GC.WaitForPendingFinalizers();

    // Assert
    Assert.Null(subscription.OnNotificationAsync);
}